1. Field of the Invention
The present invention relates to a nickel hydroxide active material for use in an alkaline storage cell such as a nickel-hydrogen storage cell, a nickel-cadmium storage cell, a nickel-zinc storage cell or the like and a manufacturing method of the same. More particularly, the present invention relates to a nickel hydroxide active material containing a cobalt compound as a conductive agent and a manufacturing method of the same.
2. Description of the Prior Art
In recent years, demands of a high performance storage cell have increased in rapid popularization of portable electronic and communication equipments. In the field of alkaline storage cells, there have been proposed various improvements of a nickel hydroxide active material adapted for use in the alkaline storage cell. For example, Japanese Patent Laid-open Publication No. 59-16269 discloses a method for forming nickel hydroxide with oxidizer in a higher order state. When the nickel hydroxide is formed in a higher order state, nickel hydroxide powder of higher density can be obtained. In the case that the nickel hydroxide powder of higher density is used as an active material, the density of the active material in a substrate of a nickel positive electrode can be increased to enhance the energy density of the nickel positive electrode.
Although the foregoing method is theoretically effective to increase the capacity of the nickel positive electrode, it is difficult to obtain oxy-nickel hydroxide of the .beta.-type (.beta.-NiOOH: 19.8 cm.sup.3 /mol) in a crystal state of higher density in volume desirable in electrochemical characterstic. As a result, oxy-nickel hydroxide of the .gamma.-type (.gamma.-NiOOH: 30.6 cm.sup.3 /mol) in a crystal state of lower density in volume undesired as an active material of the nickel positive electrode is produced, and the nickel hydroxide particles are oxidized only at their surfaces. It is, therefore, difficult to adjust the oxidation degree of nickel hydroxide particles. In addition, the oxidizer remains in the nickel hydroxide active material, causing undesired influence to the performance of the storage cell.
To avoid the problems discussed above, there has been proposed a method of electrochemically oxidizing the nickel hydroxide particles under the presence of alkaline solution, wherein a conductive substrate such as foamed nickel is filled with nickel hydroxide and a conductive additive such as a cobalt compound and is charged and discharged in a condition where it has been immersed in an amount of electrolyte. This method is useful for increasing the capacity of the positive electrode without remaining the oxidizer therein. However, after the conductive substrate filled with the nickel hydroxide and conductive additive was oxidized and reduced, the active material is apt to be removed when the conductive substrate is subjected to a washing process for removal of alkali. For this reason, the manufacturing process of the positive electrode becomes complicated. Additionally, the strength of the conductive substrate is decreased during the charge-discharge process.
Although the capacity of the alkaline cell can be increased by electrochemically oxidizing the nickel hydroxide in a powder condition, electrochemical oxidation of the nickel hydroxide may not be effected in a conventional manner since the conductivity of nickel hydroxide itself is poor. For this reason, the inventors have proposed a method for enhancing the conductivity of nickel hydroxide is disclosed in Japanese Patent Laid-open Publication Nos. 8-148145 and 8-148146. In the method disclosed in Japanese Patent Laid-open Publication Nos. 8-148145 and 8-148146, cobalt hydroxide was precipitated on the surface of nickel hydroxide particles and subjected to heat treatment under the presence of alkaline solution so that the cobalt hydroxide is formed in a higher order state. The cobalt compound formed on the surfaces of nickel hydroxide particles is superior in conductivity and is closely mixed with the nickel hydroxide particles at molecular level to effect smooth transfer of electron in electrochemical reaction. This is useful to provide a high performance storage cell.
In the foregoing methods, however, the nickel hydroxide may not be formed in a higher order state. As a result, the density in volume of nickel hydroxide is small, and an amount of the active material filled in a conductive substrate of a specific volume becomes small in comparison with an amount of an active material of higher density in volume. Accordingly, the capacity of an alkaline storage cell using the active material may not be increased. In case the nickel hydroxide may not be formed in a higher order state, it is required to form a specific amount of discharge reserve on a negative electrode. This results in decrease of utilization factor of the active material in the negative electrode.